The topics of present study are the establishment of in vitro systems for DNA synthesis in semi-conservative fashion (colicin El), in rolling circle model (human hepatitis B viral genome) and reverse transcription of viral RNA (RNA tumor viruses). Electron microscopy and sedimentation analysis of DNA synthesized in vitro showed a high grade of fidelity to in vivo synthesis. The initiation of DNA synthesis depends on the synthesis of stable RNA primer at the specific site of colicin El DNA molecule. Factors required for initiation and subsequent elongation of deoxypolynucleotide chains and the effect of carcinogens and mutagens are under investigation.